The present invention relates to a glass substrate and a method of manufacturing a glass substrate, and more particularly, to a glass substrate and manufacturing method for a magnetic disk, which has a fine textured surface.
Magnetic disks for use in hard disk drives are typically formed with a fine textured surface (pits and projections) for preventing adhesion between a head and the magnetic disk surfaces. In recent years, finer textures have been desired in order to increase the density of magnetic recording. However, using commercially practiced film texture methods and mechanical texture methods it is difficult to form the required fine textured surfaces.
Some techniques for forming a texture on the surfaces of substrates through etching are known. For example, Japanese Unexamined Patent Publication No. Hei 5-314456 discloses a technique for forming a texture by etching a glass substrate in a mixed solution of hydrofluoric acid and potassium fluoride. Japanese Unexamined Patent Publication No. Hei 10-134348 discloses a technique for forming a magnetic layer and a protective layer on a glass substrate in sequence, and then forming a texture on the surfaces of the protective layer through etching. The texture formed by the above prior art has a maximum surface roughness Rmax of 30 angstroms.
Japanese Unexamined Patent Publication No. Hei 7-153059 discloses a technique for forming a texture directly on surfaces of a glass substrate by etching the glass substrate in a hydrofluoric acid-potassium fluoride mixed solution. The height of projections of the resulting texture ranges from 130 to 400 angstroms when measured by a contact needle type surface roughness tester (trade name: Taristep, manufactured by Lank Taylor Bobson Co.).
However, the glass substrates fabricated by the prior art may sometimes have localized areas with higher projections than the average height of projections, which is known as an asperity. If a magnetic disk is manufactured using the glass substrate having such asperity, the resulting magnetic disk may experience head crash. Such a magnetic disk is susceptible to a gliding error, which may arise when a magnetic head hits an asperity in its movement (gliding). Thus, the asperity reduces production yields of the magnetic disks.
Also, even if glass substrates having the same composition are etched under the same conditions, the roughness and configuration of the fine textured surface cannot be consistently reproduced. Such unstable reproducibility makes it difficult to reliably mass-produce glass substrates having uniformly fine textures.
In recent years, majority type of hard disk drives is a ramped loading type. The magnetic head, which used in the ramped loading type, is located above the disk surface spaced by about 10 nm. However, the design of magnetic heads differs from one hard-disk manufacturer to another, also the design of head-disk-interfaces differs. Therefore, the hard-disk manufacturers respectively require slightly different degrees of surface roughness on the magnetic disk. It is therefore desired to accurately control the roughness of the fine textured surfaces.